Non-destructive hydro-excavation or vacuum excavation techniques are widely accepted as the safest way to expose or excavate around underground services. In simple terms, a jet of water is blasted into soil to liquefy the soil and then the liquefied soil is sucked up using a vacuum truck. The process results in spoil that is mostly slurry that may be about 50:50 water and dirt. The slurry is waste and needs to be disposed of. In many countries and regions the slurry is considered to be contaminated and therefore there are limitations on how the disposal can be handled. In many cases the slurry must be transported to a suitable location for dumping. The suitable location is often a significant distance from the excavation site. This represents a significant cost in transport but also in lost productivity of the vacuum truck.
There is significant benefit if the amount of water in the spoil can be reduced. Firstly, the volume of material to be transported is reduced thus reducing the transport asset requirement. Secondly, the partly dry waste is easier to handle and less problematic for disposal.
At present the approach commonly used is to dump the spoil in a holding pond and allow slow dewatering by settling. The partly dry slurry is then re-handled to truck it to a permanent disposal site. This process is not available in many places as the water is considered to be contaminated and therefore cannot be allowed to run into the surrounding environment from a holding pond. Disposal issues associated with hydro-excavation are proving to be a negative impost on the rate of take up worldwide. Disposal of spoil is a significant time and cost issue.
The Inventors are aware of a number of devices that are used for dividing slurries or sludge into different fractions or for removing larger contaminants from sludge or slurries. For instance, Chinese patent application 104710088 describes a sludge treatment system that sieves sludge to remove large contaminants. It does not remove water or further improve the sludge for handling. Chinese patent application number 202688158 describes a system for treating oil sludge to remove large-size solid particles before an homogenization treatment. Korean patent number KR101365734 is similar to the two Chinese applications but aims to reduce moisture content by heating.
In many countries disposal of spoil requires a license, but fewer facilities are able to meet stringent requirements being imposed. Consequently, disposal facilities are:                Located further and further away from the CBD (City Centre);        Subject to government waste levies in some states and territories;        Becoming highly regulated and prohibitively expensive;        Operating under limited hours.        
This has a number of implications including:                Increased Hydro-excavation costs;        Higher disposal costs;        Loss of productivity with increased off site time being paid for the disposal process;        Loss of production of our client's labour and equipment (time lost during        Hydro Vac unit off-site disposal).        
There is a need for a solution that addresses at least some of these problems. In any event, there is benefit if the water can be removed from the spoil at or near the excavation site as this allows for reuse of the water, thus further reducing excavation costs. It is also useful if the Spoil Treatment Plant is easily relocatable so it can be positioned as close to the source of the spoil as practical.